This invention relates generally to thermostatic switches, and more particularly to a thermostat having a uniquely constructed trip free, manual reset feature.
The thermostat of the present invention is designed to be used in appliances, photocopying machines, computers, and the like. Most thermostats presently available for such use are capable of being reset, and typically include a manually operable plunger that is engageable with an actuating member which is in turn engageable with a bimetallic disc. Bimetallic discs are well-known in the art of thermostatic switches and are designed to flex or move when the environment in which the thermostat is placed reaches a predetermined temperature. In prior thermostats, resetting of the circuit in the thermostat was accomplished by pressing a reset element in the thermostat to move the actuating member into engagement with the disc and completing the circuit.
One problem associated with these types of thermostats is that often it is not desirable to return the movable contact member to the closed position and establish electrical communication through the thermostat until the disc has returned to its original (or switch-closed) configuration. More specifically, in instances where the temperature does not go below the predetermined temperature, the movable contact member is prematurely returned to the switch-closed position which in some instances results in malfunction of the thermostat or other temperature-related hazard. Oftentimes it is desirable to open or close an electric circuit and retain it in such condition until the thermostat is manually reset and only after the temperature has returned to an acceptable level (i.e., below the predetermined temperature).
Several thermostats have been designed to address this problem. One such thermostat is disclosed in U.S. Pat. No. 4,039,991 to Bucheister, which is assigned to the assignee of the present application. This patent discloses a thermostat having a lock device that is engageable with a movable contact member in the switch-open position. The lock device comprises a spring lever which overlies the contact member and is secured to the housing by a rivet. A latch finger having an inclined surface defining a notch is attached to the spring lever. The notch is located so that the latch finger engages the contact member for retaining and maintaining the contact member in a switch-open position upon the bimetallic disc flexing in response to increased temperature.
It should be noted that the reset feature of Bucheister's thermostat, while sufficiently suitable from a performance standpoint, is relatively complex in design and expensive to manufacture. This is due to the additional component parts required to achieve the lock device, i.e., the spring lever, rivet, and latch finger, and the additional assembly time.
"Trip free" thermostats are also well-known in the art. Reference is made to U.S. Pat. Nos. 3,621,434 to Gerich, 3,832,667 to Blanton, 5,003,282 to Boulanger, and 5,157,370 to Engelbach et al. as representative prior art disclosing trip free thermostats. In this regard, the Boulanger patent is most pertinent reference in the art of the present invention. Boulanger discloses a thermostat having a housing with a push button extending axially through an opening formed on one end of the housing. Two normally closed contacts are disposed within the housing, one of the contacts being stationary while the other contact is movable on a spring-biased arm. Boulanger further provides a bimetallic disc on the other end of the housing, the disc flexing inwardly when the thermostat is subject to a temperature higher than a predetermined temperature. A spring is disposed between the disc and the arm, and, when the thermostat is subject to a temperature higher than the predetermined temperature, the spring moves the arm so as to disengage the contacts.
To reset Boulanger's thermostat, the push button is moved toward the disc which rotates the spring member about a pivot, thereby resetting the disc and pushing the end portion of the spring against the arm having the movable contact to maintain the contacts open. The contacts remain open until the push button is released which allows the spring to rotate away from the carrier which in turn allows the contacts to engage one another once again.
One disadvantage associated with Boulanger's approach is that it is relatively complicated in design since there are two springs which require precise machining and calibration in order to perform properly. This complicacy results in the thermostat being relatively expensive to manufacture, and difficult to assembly since the springs require exact placement within the housing.